Ammonia is a widely used chemical with many applications. One specific application is as reductant for selective catalytic reduction (SCR) of NOx in exhaust gas from combustion processes.
For most applications, and in particular in automotive applications, the storage of ammonia in the form of a pressurized liquid in a vessel is too hazardous. Urea is a safe, but an indirect and impractical method for mobile transport of ammonia since it requires urea to be transformed into ammonia by a process involving spray, evaporation, thermolysis and hydrolysis ((NH2)2CO+H2O→NH3+CO2), which is difficult to achieve under driving conditions with low engine load or cold weather.
A storage method involving ad- or absorption of molecular ammonia in a solid can circumvent the safety hazard of anhydrous liquid ammonia and eliminate the problem with decomposition of a liquid reductant.
Metal ammine salts are ammonia absorbing and desorbing materials, which can be used as solid storage media for ammonia (see, e.g. WO 2006/012903 A2), which in turn, as mentioned above, may be used as the reductant in selective catalytic reduction to reduce NOx emissions.
For the use as ammonia source for automotive NOx reduction, the demands for the functionality are tightly linked to emission legislation and dynamic operation under real driving conditions. Ammonia Storage and Delivery Systems—hereafter abbreviated ASDS—have to be able to deliver ammonia shortly after engine start to enable vehicle certification according to e.g. European and US driving cycles.
WO 2008/077652 A2 discloses a method and device that allows for rapid availability of ammonia from a solid storage material by having a system with two main functionalities: A small, operational unit heated by e.g. electricity and a larger ammonia storage unit that is used as a source to carry out on-board resaturation of the small unit. The method and device can have at least two types of configurations:                if the material in the smaller unit has a higher binding strength for ammonia absorption than the material in the large unit, then the smaller unit can passively absorb ammonia from the big tank after operation—even if the larger unit is not heated.        If the materials in the two tanks are the same, then the larger tank is equipped with means for heating that provides the opportunity to reach suitable desorption pressure from the main tank to resaturate the smaller unit.        
What remains for optimizing such a system is to be able to determine when to carry out the resaturation of the smaller storage unit—or more directly a methodology that gives the same type of functionality as a liquid level sensor in a tank. One way of finding out that a (smaller) storage unit requires resaturation (from a larger unit) is when it is empty. Materials that absorb ammonia typically have well-defined relationship between temperature and desorption pressure and if a required desorption pressure cannot be established at a certain temperature, then the storage material is depleted of ammonia—just like one cannot create pressure of steam in a boiler if the unit does not contain any water
WO 2009/156204 A1 discloses a system similar to WO 2008/077652 A2 but with an added feature that the small storage unit is determined as empty by using pressure measurement and the temperature of the storage unit. However, it is not clear whether such a method works really satisfactory
Therefore, there is a desire for a new method enabling the degree of saturation of ammonia in a container loaded with a material capable of absorption and desorption of ammonia to be determined or estimated. This method should preferably not introduce new, costly equipment in the system.